This invention relates to a position control apparatus and method, and more particularly to an apparatus and method for sensing and controlling the position of a movable machine element.
Various systems are known in the art for controlling the position of a movable machine element, such as a table or tool of a machine tool. Examples of these conventional systems are illustrated in FIGS. 1 through 3. In FIG. 1, the shaft of a motor MT for driving a movable machine element ME has a rotary encoder (pulse generator) REC attached thereto for generating feedback pulses Pf of a frequency proportional to the rotational speed of the motor. The feedback pulses Pf are applied to a position control circuit PCC which also receives distributed pulses Pi, referred to as command pulses, generated by a pulse distributing circuit PDC in response to an externally applied command. The position control circuit PCC computes the difference between the command pulses Pi and feedback pulses Pf and generates a speed command signal Vc commensurate with the computed difference. The feedback pulses Pf from the rotary encoder also enter a speed detecting circuit VD for producing an actual speed signal Va proportional thereto, the signal Va indicating the actual rotational speed of the motor MT. A speed control circuit VCC supplies the motor MT with a signal commensurate with the difference between the commanded speed and actual speed to drive the motor into rotation, thereby controlling the position of the machine element ME. The apparatus of FIG. 1 thus requires only a single detector, namely the rotary encoder REC, and can be realized with an inexpensive servo system. The arrangement of FIG. 1 is disadvantageous, however, in that only speed control can be performed, making it impossible to achieve accurate positional control.
In the arrangement of FIG. 2, a tachogenerator TCM is specially provided to serve as a speed detector, and a position detector IDS, such as an Inductsyn or optical scale, is employed to detect the position of the movable machine element ME and to produce a signal indicative of the position. With this apparatus, highly accurate and responsive positional control is achieved because the position signal produced by the detector IDS is derived directly from the object being controlled, namely the movable machine element ME. A disadvantage, however, is the comparatively high cost entailed by the use of two detectors, these being the tachogenerator TCM and position detector IDS. Furthermore, although the arrangement provides positional information relating to the movable machine element, no such information is available regarding the motor shaft. Another shortcoming is that a non-linear characteristic of the movable element appears in the position control loop when there is backlash in the mechanical mechanism of the movable element, making it unadvisable to raise the gain of the position control loop.
FIG. 3 illustrates a position control system capable of performing so-called hybrid control, this circuit can be found in Fanuc Ltd.'s System 9--Model A and is shown in detail on p. 23 and described in conjunction therewith in FANUC SYSTEM 9--MODEL A (Supplemental Specification) obtainable from Fanuc, Ltd., Japan. The chief advantage of which is that the gain of the position control loop can be raised when desired. The system includes a tachogenerator TCM for detecting motor speed, a detector such as a resolver RSV for detecting motor position, and a detector IDS such as an Inductsyn or optical scale for detecting the position of the movable element ME of the machine. An arithmetic unit ASU computes the difference PE between a motor position signal MP produced by the resolver RSV and a signal TP, produced by the position detector IDS, indicating the position of the movable element, and feeds a signal indicative of PE through a first-order lag element FDC for addition to a signal CP indicative of a commanded position. A signal CP' resulting from the addition operation is compared with the motor position signal MP, and the difference between the two signals is amplified by an amplifier AF to produce the command speed signal Vc. The speed control circuit VCC produces a signal corresponding to the difference between the commanded speed Vc and the actual motor speed Va, and applies the signal to the motor MT. The motor MT is driven by the signal to transport and, hence, control the movable machine element ME. The foregoing arrangement permits the system gain to be raised and makes it possible to perform highly responsive, accurate positional control. The drawback, however, is a higher cost owing to use of the three detectors TCM, RSV and IDS.